Origin and evolution of ferroelectricity in the layered rare-earth-titanate, R2R_2Ti2_2O7_{7}, Lichtenberg phases

We report a systematic first-principles study based on density functional theory (DFT) of the structural and ferroelectric properties of the $R_2$Ti$_2$O$_{7}$ perovskite-related oxides with $R=$ La, Ce, Pr, and Nd. We show that, in all cases, the ferroelectric distortion to the ground-state polar $P2_1$ structure from its parent centrosymmetric $P2_1/m$ phase is driven by a single polar soft mode consisting of rotations and tilts of the TiO$_6$ octahedra combined with displacements of the $R$ ions. A secondary centrosymmetric distortion, which is stable in the parent structure, contributes substantially to the energy lowering of the ground state. We evaluate the trends in structure and polarization across the series as a function of $R$ and reconcile discrepancies in reported values of polarization in the literature. Our results confirm that the family of $R_2$Ti$_2$O$_{7}$ materials belong to the class of proper geometric ferroelectrics.Comment: 15 pages, 12 figures, 7 table

Elasticity of Diamond at High Pressures and Temperatures

We combine density functional theory within the local density approximation, the quasiharmonic approximation, and vibrational density of states to calculate single crystal elastic constants, and bulk and shear moduli of diamond at simultaneous high pressures and temperatures in the ranges of 0-500 GPa and 0-4800 K. Comparison with experimental values at ambient pressure and high temperature shows an excellent agreement for the first time with our first-principles results validating our method. We show that the anisotropy factor of diamond increases to 40% at high pressures and becomes temperature independent.Comment: 10 pages, 3 figures, 1 tabl

Thermoelastic Properties of Ringwoodite [Fe_x,Mg_(1-x)]_2SiO_4: Its Relationship to the 520 km Seismic Discontinuity

We combine density functional theory (DFT) within the local density approximation (LDA), the quasiharmonic approximation (QHA), and a model vibrational density of states (VDoS) to calculate elastic moduli and sound velocities of gamma-[Fe_x,Mg_(1-x)]_2SiO_4 (ringwoodite), the most abundant mineral of the lower Earth's transition zone (TZ). Comparison with experimental values at room-temperature and high pressure or ambient-pressure and high temperature shows good agreement with our first-principles findings. Then, we investigate the contrasts associated with the beta-to-gamma-[Fe_x,Mg_(1-x)]_2SiO_4 transformation at pressures and temperatures relevant to the TZ. This information offers clearly defined reference values to advance the understanding of the nature of the 520 km seismic discontinuity.Comment: 29 pages, 6 figures, 2 tables. Under Revie

Evidence for a pressure-induced spin transition in olivine-type LiFePO4\mathrm{LiFePO_{4}} triphylite

We present a combination of first-principles and experimental results regarding the structural and magnetic properties of olivine-type $\mathrm{LiFePO_{4}}$ under pressure. Our investigations indicate that the starting $Pbnm$ phase of $\mathrm{LiFePO_{4}}$ persists up to 70 GPa. Further compression leads to an isostructural transition in the pressure range of 70–75 GPa, inconsistent with a former theoretical study. Considering our first-principles prediction for a high-spin to low-spin transition of $\mathrm{Fe^{2+}}$ close to 72 GPa, we attribute the experimentally observed isostructural transition to a change in the spin state of $\mathrm{Fe^{2+}}$ in $\mathrm{LiFePO_{4}}$. Compared to relevant Fe-bearing minerals, $\mathrm{LiFePO_{4}}$ exhibits the largest onset pressure for a pressure-induced spin state transition